Such a control circuit is used, for example, for controlling and monitoring a central locking means of a motor vehicle. Such a control means can be used quite generally for controlling and monitoring a so-called automatic state apparatus which is capable of producing a predetermined number of states, changes from one state to another state on the basis of actual states and input variables and produces output signals in doing so.
DE 42 21 142 A1 discloses a central locking system for a motor vehicle, comprising a transmitter incorporated in a door key and a receiver accommodated in the motor vehicle. By means of the transmitter, a code is transmitted that is decoded by the receiver and causes actuation of the central locking system when the correct code has been transmitted. Transmitter and receiver thus constitute a remote control means. For permitting the latter to operate selectively either with radio frequencies or with light frequencies, there are provided on the transmitter side both an HF oscillator and a light wave oscillator whose HF carrier and light wave, respectively, can each be modulated with the code word on the transmitter side, and on the receiver side there are provided both a HF detector and a light wave detector whose output signals are fed to a common decoder means.
U.S. Pat. No. 417 discloses a remotely controllable central locking system for a motor vehicle, with the receiver thereof, which is arranged within the motor vehicle, being periodically turned on and off in order to reduce the overall power consumption. For making sure that the central locking system is definitely responsive in case of transmission of a code signal from a transmitter, the code pulse sequence on the transmitter side is preceded by a leader pulse having a duration that is longer than the time distance between two successive on-state intervals of the receiver. In this manner, the receiver is safely activated by the leader pulse, so as to be able to receive and process the code pulse sequence thereafter. To this end, the receiver is provided with a clock pulse generator delivering clock pulses corresponding to the on-state intervals of the receiver to a first input of an AND circuit. A second input of the AND circuit is fed with pulses that are received by the transmitter and shaped. If a pulse from the transmitter is received by the timer during a clock pulse, the then created output signal of the AND circuit triggers a monostable multivibrator, the output signal of which turns on a power supply of the receiver for a predetermined period of time that is at least as long as the code pulse sequence transmitted by the transmitter subsequently to a leader pulse. When no pulse from the transmitter has been received during a clock pulse, the power supply of the receiver is turned on only for the particular duration of the clock pulse.
EP 0 457 964 A1 reveals a remote operating system for controlling additional apparatus for vehicles, whose receiver arranged in the vehicle is periodically turned on and off in order to reduce the average power consumption of the receiver. During a transmission operation, the transmitter is turned on each time for a period of time of such duration that at least one on-state interval of the receiver is present therein so that the receiver can definitely be responsive to a transmission operation.
DE 43 02 232 A1 discloses an apparatus for operating a microprocessor, by means of which the microprocessor can be operated in an active and in an inactive operating state so as to reduce the load acting on the battery supplying current to the microprocessor. In the inactive state, the microprocessor may be brought to the active state either by a wake-up signal of a watchdog provided in the microprocessor or by an external wake-up signal issued periodically by an external oscillator. The external oscillator is composed with two CMOS inverters.
A conventional control circuit of the type indicated at the outset comprises a control means, which may be a microcontroller, and a main oscillator delivering a clock signal for operation of the control means. In addition thereto, such a control circuit may contain a state monitoring means through which the states of predetermined electrical means, such as electrical switching contacts, sensors and/or detectors, can be monitored and state signals representing the respective states can be delivered to the control means.
Due to the high clock frequencies that may be employed by digital control means of modem nature, in particular in the form of the already mentioned microcontrollers, quartz oscillators are used having oscillation frequencies in the MHz range. Both such control means as well as such oscillators consume relatively much power, which may turn out problematic for example in such cases in which the means controlled by the control circuit is not required for long periods of time. If such a control circuit is used, for example, for controlling a central locking system of a motor vehicle, it may happen that the control circuit is not being used for a long period of time, for example when the motor vehicle is not in use for days, weeks or even months. In order to avoid that the electrical source of energy, in the example mentioned a motor vehicle battery, is subjected to undesirable loads, it is known to switch the control circuit, when its control function is not required for a longer period of time, to a current-saving waiting or standby mode of operation in which control circuit components with relatively high power consumption, such as the control means and the oscillator, are turned off.
In the standby mode, only such parts of the control circuit are kept in the on-state mode which serve for state control of electrical means, such as sensors, detectors and switch contacts. In this manner, it is possible to determine when a need for control by the control unit arises again, so as to be able to reset the control circuit to full operation thereof in case of such determination. Control circuit parts that are deactivated during standby operation are thus put into operation again.
For reasons of functional safety, the control circuit is also reset to full operation for a short wake-up period each when no control necessity is present. Such temporary resetting to full operation usually takes place periodically. For example, after standby periods of a duration of several seconds each, resetting to full operation takes place for a wake-up period of several milliseconds each. In this example, the control circuit is in full operation only in the range of some few percent of the total time, and the remaining time in the standby mode. The average power consumption by the control circuit parts with noticeable power consumption is correspondingly reduced to some few percent of the power consumption that would arise if the control circuit were kept in full operation at all times.
For controlling the control circuit parts held in the on-state during the standby mode as well as for controlling the alternating standby periods and periods of full operation, an oscillator is required for making available clock signals necessary therefor, and the frequency of these clock signals may be considerably lower than that of the clock signals fed from the quartz oscillator to the control means. Due to the fact that the quartz oscillator is turned off during the standby mode, this known control circuit uses, in addition to the quartz oscillator serving as main oscillator, a second oscillator serving as a standby oscillator that is permanently in operation and has a considerably lower oscillation frequency than the main oscillator and a considerably lower power consumption than the main oscillator. In conventional manner, for example an RC oscillator or an IC oscillator is employed as standby oscillator, with a capacitor thereof being periodically charged and discharged with the aid of a current source and a switch.
Such standby oscillators involve problems in so far as the frequency stability thereof is not very good.